how many players are active?

I just played two seasons. I feel only 20% players are still active. How about higher level?

You'll probably find a higher percentage of active players the higher the level.

You'll find that as you go up in levels, there are less and less inactive managers. In the lower levels (up to about level 4 or 5), managers have started and then left the game, but their teams are still there. As you get higher, the managers are more dedicated, hence less abandoned teams and tougher matches.